The source code for optiboot is at http://code.google.com/p/optibootThere was a "big sync-up" back in October of 2011 (prior to 1.0), so that that code and the optiboot included in the arduino distribution were identical. This was Optiboot version 4.4. Since then there have been a number of change that have been made only in the optiboot project, because they have not resulted in any change in the behavior (or code) of optiboot on any of the official arduino platforms. They MIGHT be relevant if you were building some custom platform; for example one of the changes changed the way that the uart bit rate divisor was calculated, especially for lower bitrates.The changes are recorded by code.google.com, and mostly also in the source code itself: